Method and Apparatus for Adjustment of Current Through a Magnetoresistive Tunnel Junction (MTJ) Based on Temperature Fluctuations

ABSTRACT

A non-volatile memory system includes a first circuit and a second circuit both coupled to a magnetoresistance tunnel junction (MTJ) cell to substantially reduce the level of current flowing through the MTJ with rise in temperature, as experienced by the MTJ. The first circuit is operable to adjust a slope of a curve representing current as a function of temperature and the second circuit is operable to adjust a value of the current level through the MTJ to maintain current constant or to reduce current when the temperature increases. This way sufficient current is provided for the MTJ at different temperatures to prevent write failure, over programming, MTJ damage and waste of current.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of the commonly assignedapplication bearing Ser. No. 14/824,982 filed on Aug. 12, 2015 byAbedifard et al. and entitled “METHOD AND APPARATUS FOR ADJUSTMENT OFCURRENT THROUGH A MAGNETORESISTIVE TUNNEL JUNCTION (MTJ) BASED ONTEMPERATURE FLUCTUATIONS,” the content of which, including thespecification, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates generally to magnetoresistive tunnel junctions(MTJs) and particularly to reliability thereof.

Description of the Prior Art

Magnetoresistive tunnel junctions (MTJs) show future promise inreplacing the position today's random access memories (RAMs) enjoy for anumber of reasons among which is their considerably smaller size andgreater speed. However, challenges remain in producing reliable MTJs,particularly in volumes. Designers and manufacturers struggle with theconsistent and reliable programmability and sensing of MTJs. Their smallform factor is of no help and rather causes challenges such as providingsufficient voltage and current for proper operability of MTJs yet notexceeding the level of current to the point of causing permanent damage.This may seem like a trivial task but in reality, when working on a verysmall level, reaching a sweet spot range can be a challenging task inand of itself.

As a fall out of MTJ's programming characteristics, current requirementsdecrease when temperature rises. When programmed with excessive current,MTJs are placed in danger of being permanently damaged.

There is a phenomenon known as “hopback” that occurs due to intolerablecurrent levels where programming the MTJ to a certain logical state thatcauses the MTJ to take on a high resistance results in the programmingof the MTJ to a state represented by a low resistance instead. Clearly,this outcome is unacceptable. Another outcome of higher-than-necessarycurrent flow through a MTJ is unnecessary power consumption. One way ofovercoming the problems associated with an undesirable increase incurrent at high temperatures is to control the level of current throughthe MTJ and compensate at different temperatures. But to do so requiresthe use of a current source that uses considerably higher voltages thanthat required by MTJ and is therefore impractical.

Thus, there is a need for reliable programming and sensing of MTJs atdifferent temperatures.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa method and a corresponding structure for a magnetic memory systemincluding magnetic tunnel junctions (MTJs) and structures and methodsfor programming the same.

Briefly, a non-volatile memory system comprises a first circuit and asecond circuit both coupled to a magnetoresistance tunnel junction (MTJ)cell to substantially compensate the level of current flowing throughthe MTJ over variances in temperature, as experienced by the MTJ. Thefirst circuit is operable to adjust a slope of a curve representingcurrent as a function of temperature and the second circuit is operableto adjust a value of the current level through the MTJ to compensate thecurrent or basically reduce current when temperature increases.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the preferred embodiments illustratedin the several figures of the drawing.

IN THE DRAWINGS

FIG. 1 shows a graph 10 of change in programming current flow, through amagneto-resistive tunnel junction (MTJ), as shown in the y-axis, overthe change in temperature experienced by the same MTJ, as shown in thex-axis.

FIG. 2 shows a circuit 20 coupled to the MTJ 22 for controlling thecurrent that flows through the MTJ 22, in accordance with an embodimentof the invention.

FIG. 3 shows a circuit 300 for adjusting the current flow through a MTJ,in accordance with another embodiment of the invention.

FIG. 4 shows a circuit 400 for adjusting the current flow through atransistor, in accordance with yet another embodiment of the invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which is shownby way of illustration of the specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized because structural changes may be madewithout departing from the scope of the present invention. It should benoted that the figures discussed herein are not drawn to scale andthicknesses of lines are not indicative of actual sizes.

Current is important for programming the MTJ. A current source providescurrent despite voltage and seems to be a viable candidate forcontrolling current through the MTJ but this is actually an impracticalapproach because a lot of voltage is required to implement a currentsource.

In accordance with various embodiments and methods of the invention,current is controlled or adjusted to compensate for the rise intemperature so as to prevent hopback and damage to the MTJ.

FIG. 1 shows a graph 10 of change in current flow, through amagneto-resistive tunnel junction (MTJ), as shown in the y-axis, overthe change in temperature experienced by the same MTJ, as shown in thex-axis. The graph 10 shows four curves in particular, curves 12-18, oneof which, i.e. curve 18, represents the relationship between current andtemperature of the MTJ under ideal conditions with current remainingconstant as temperature increases. Clearly, this ideal condition isunrealistic and impractical. Rather and in accordance with a method ofthe invention is providing current for the MTJ that is sufficient forprogramming it at all temperatures.

In the graph 10, curve 12 shows the behavior of current through the MTJas temperature of the MTJ increases. A substantial drop in programmingcurrent is experienced by the MTJ as it becomes hotter. There are manyenvironments in which a memory system whose memory is made of orincludes MTJs and that experiences temperature variations. To name asimple example, oftentimes memories in computers or servers experience asignificant increase in temperature due to the operation of the sheernumber of components of the computer/server. The curve 12 showsgenerally, the behavior of MTJs with temperature.

In an ideal world, curve 12 should be precisely tracked but that is nota possibility. To maintain the current flow through a MTJ experiencing arise in temperature as constant as possible, the inventor has devisedmethods and embodiments for adjusting the slope as well as the currentlevel through the MTJ.

It is understood that the methods and embodiments of the invention forprogramming of (or writing to) an MTJ are applicable to practically anytype of MTJ and further applicable to any programmable non-volatile andvariable resistive memory element.

Curve 14 is intended to show the result of the current level through theMTJ, as temperature increases, where an adjustment to the current levelis made using the various embodiments of the invention. Arrows 21, inFIG. 1, are intended to show the change in the curve 12 to the right orleft of the curve 12 when current is adjusted per various embodiments ofthe invention. Note that the slope of the curve 12 remains substantiallyconstant as current is adjusted.

Curve 16 similarly shows current vs. temperature but it is the slope ofthis curve and not the shift up or down that results when an adjustmentis made pursuant to various embodiments of the invention. While notshown in graph 10 for the sake of clarity, the slope of curve 16 changesby adjustment of resistor values of resistors 36 and 38 in FIG. 2 orresistors 304 and 306 in FIG. 3. That is, representing this factor by‘n’ in FIG. 2, the slope of curve 16 alters with changes to ‘n’ asindicated in and discussed relative to the equation below with referenceto FIG. 2. ‘N’ represents R1/(R2−R1) where R1 and R2 each represent aresistance value. As will be further discussed below, when a referencevoltage, Vref, changes by factor ‘n’, current also changes by a factorof ‘n’.

The following circuits, among a slew of others too numerous to listhere, adjust the current level of current flowing through the MTJthereby shifting of the current-to-temperature curve up and down whileadjusting Vref by a factor of ‘n’ thereby causing the slope of thecurrent-to-temperature curve to change, all in an attempt to bring thecurve as close to the curve 18 as possible.

FIG. 2 shows a circuit 20 coupled to the MTJ 22 for controlling thecurrent that flows through the MTJ 22, in accordance with an embodimentof the invention. The circuit 20 is shown to include transistors 24 and26 with the drain of the transistor 24 coupled to Vdd and its gatecoupled to the current mirror 28, also a part of the circuit 20. Thesource of the transistor 24 is shown coupled to the drain of thetransistor 26 as well as to its gate therefor making the transistor 24 acurrent source that sinks current with the same current values as thatof the current through the transistors 28. Similarly, the transistor26's gate and drain are coupled together to provide constant current.The gates of transistors 30, a part of circuit 20, control the level ofcurrent flowing through the diodes 36 and 38 by the operation of theamplifier 44.

The circuit 20 is further shown to include resistors (R1) 32 and (R2)34, along with the diodes 36 and 38, operational amplifier 44, theswitch circuit 40 and the current mirrors circuit 42. Each of thecircuits 40 and 42 include a series of transistors with gates of eachthe transistors of the circuit 40 coupled to the source of thecorresponding transistor except for one transistor whose gate isgrounded. The gates of each of the transistors of the circuit 42 areshown coupled to the current source 30, the drain of each of thetransistors of the circuit 42 is shown coupled to the MTJ 22 and each ofthe sources of the transistors of circuit 42 is shown coupled to arespective transistor of the circuit 40. More particularly, the sourceof each of the transistors of the circuit 42 is shown coupled to thedrain of a respective transistor of the circuit 40. It should be notedthat while each of the circuits 40 and 42 is shown to include fourtransistors, any suitable number of transistors may be employed in each.

Current mirror 28 and the current source 30 are shown coupled together.Current mirror 28 is also shown coupled to Vdd and the current course 30is shown coupled to the operational amplifier 44 and to the resistors 32and 34, thus, on the same end, each of the resistors 32 and 34 iscoupled to the amplifier 44 while at an opposite end, each of theresistors 32 and 34 is coupled to a respective diode of the diodes 36and 38. Namely, the resistor 32 is shown coupled to the diode 36 and theresistor 34 is shown coupled to the diode 38. Each of the diodes 36 and38, at an opposite end, is coupled to ground, as is the source of thetransistor 26. It is understood that ground represents a voltage that issubstantially zero and Vdd represents a higher voltage than zero andtypically the highest voltage supplied to the circuit 20.

The amplifier 44 receives as another input, Vdd, and outputs a referencevoltage, Vref, that is coupled to the current source 30 and the circuit42. Accordingly, the output of the amplifier 44 is coupled to an end ofthe MTJ 22, through the circuit 42, while an opposite end of the MTJ 22is coupled to ground.

Vref=Vbes+n*(Vbes−Vbel)  Eq. (1)

In Eq. (1), ‘Vref’ represents the voltage at node 50. ‘Vbes’ representsthe voltage across diode 36 (the smaller diode) and ‘Vbel’ representsthe voltage across diode 38 (the larger diode, in this case times largerthan diode 36). ‘N’ represents R1/(R2−R1). As represented by Eq. (1),‘n’ is used to control the slope of the curves shown in FIG. 1.

Diode 34 has a size that is ‘n’ times the size of diode 32. Whentemperature across MTJ 22 rises, Vbes decreases but (Vbes−Vbel)increases because in light of the diodes' size difference, the voltageacross the diode 36 changes more rapidly than the voltage across thediode 34.

The circuits 40 and 42 generally control the level of the currentthrough MTJ 22 while the remainder of circuit 20 controls the slope ofthe change in current through the MTJ 22 as a function of changes intemperature experienced by the MTJ 22.

The goal of circuit 20 is to control the current level through the MTJ22 such that the MTJ 22 is provided with the current level it needs tobe operational at all temperatures, i.e. follow the naturalcharacteristics of the MTJ. Accordingly, current mirror circuits 28 and30 mirror the current through the resistors R1 and R1. The currentthrough the MTJ is adjust to satisfy its current requirements atdifferent temperatures. This combination of R1 and R2 adjusts the slopeof the curve 16, for example, to a proper slope to allow for operabilityof the MTJ 22. The mirrored current from the circuits 28 and 30 isprovided as reference current, or reference voltage since voltage andcurrent are factors of each other, to the MTJ 22. Namely, resistances ofR1 and R2 define ‘n’ because n=R1/(R2−R1) and ‘n’ is multiplied by(Vbes−Vbel), as noted in Eq. (1). Therefore, R1 and R2 effectivelydetermine the slope of the curves 14 and 16 shown in FIG. 1. ResistancesR1 and R2 are therefore adjusted until the desired slope, i.e. slope ofthe curve 12, is approximately reached. R1 and R2 are generally fixed,for example set during manufacturing, and based on the characteristicsof the specific MTJ being employed.

In summary, as temperature rises, Vbes drops but (Vbes−Vbel) risesbecause Vbel drops faster than Vbes. ‘N’, which is essentially the ratioof resistances R1 and R2, is chosen such that Vref, at node 50, canincrease, decrease or stay the same, as desired. Current through the MTJ22 is controlled by measuring the voltage at a connection between aresistor and a diode, such as the node 50, and detecting a voltagechange thereof. The voltage change is based on a change in temperatureat the foregoing connection. Upon detecting the voltage change, thevoltage at the foregoing connection is digitized and converted to acurrent, which is used to operate the MTJ 22. Accordingly, during a risein temperature of the MTJ 22, the current through the MTJ 22 is reduced.

Circuit 42 on the other hand, effectively determines how much the curves14 and 16 move up and down based on the number of transistors making upthe circuit 42 and the size of these transistors. Thus, the number andsize of the transistors of circuit 42 are adjusted to the point wherethe desired current level, i.e. the amount of up and/or down of thecurves 14 and 16, is reached. The more transistors in the circuit 42,the higher the variety of currents that can be provided. The currentthrough one of the transistors of circuit 42 is proportional to thecurrent through any of the other transistors of circuit 42. Current thatis ultimately supplied to the MTJ 22 is further controlled by thecircuit 40.

Circuit 40 functions as a switch to the circuit 42. More specifically,based on the coupling of each of the gates of the transistors of circuit40, a respective one of the transistors of circuit 42 is turned on oroff. In an exemplary embodiment of the invention, coupling of the gate(B) of one of the transistors of the circuit 40 to ground, turns thetransistor immediately below it in circuit 42 ‘on’ and coupling of thesame gate (B) to Vdd, turns the transistor immediately below it incircuit 42 ‘off’, or vice versa. Each transistor may be independentlycoupled to ground or Vdd. In this manner, coupling of each transistor ofthe circuit 40 to ground or Vdd serves to turn on or off the respectivetransistor.

When turned ‘on’, the transistors of circuit 42 supply additionalcurrent to the MTJ 22. As earlier stated, based on the number oftransistors and size thereof, the current supplied to the MTJ 22 isadjusted. Depending on the coupling of each transistor of the circuit40, a corresponding transistor of the circuit 42 is turned ‘on’ or‘off’. The more transistors of circuit 42 that are turned ‘on’, the morecurrent the MTJ 22 is supplied with because turning ‘on’ a transistor ofthe circuit 42 causes extra current to flow through to the MTJ 22. When‘off’, the MTJ 22 remains unaltered. When some of the transistors areturned on and some are turned off, the level of current applied to theMTJ 22 changes. In this manner, the level of current through the MTJ 22is adjusted.

In summary, the voltage Vref goes up, stays the same or goes downdepending on the value of “n”. “n” determines the slope of the curves ofFIG. 1. The amount of current flowing through the transistors of circuit42 follows the n-determinative slope. Current is mirrored to the currentthrough the transistors of circuit 42, which can be and commonly are ofvarying sizes. For example, the four transistors may be sizes thatfactors of each other, such as 1×, 2×, 3×, and 4×. The transistors ofcircuit 40 function as switches turning on and off the transistors ofcircuit 42. Thus the transistors of circuit 42 determine how much thecurve goes up and down. In this manner, with rising temperature, currentmay be lowered.

FIG. 3 shows a circuit 300 for adjusting the current flow through a MTJ,in accordance with another embodiment of the invention. The circuit 300is shown to include a slope-adjustment circuit 302, transistors 326 and324, which are shown coupled to each other in series, switch circuit340, and current mirror circuit 342, and the MTJ 22. The circuits 340and 342 are analogous to the circuits 40 and 42, respectively. Thecircuit 300 is shown coupled to the MTJ 22.

As in FIG. 3, the transistors of the circuit 340 are coupled together inparallel, similarly, the transistors of the circuit 342 are coupledtogether in parallel. Circuit 340 is also coupled in parallel withtransistor 324, similarly, circuit 340 is coupled in parallel withtransistor 326.

The circuit 302 includes two resistors of different type, coupledtogether in parallel and coupled to the transistor 36 at a common node.At an opposite common node, the circuit 302 is coupled to the MTJ 22.The MTJ 22 is in turn coupled to the circuit 342. The transistors 324and 326 are coupled together in series.

Using the embodiment of FIG. 3, the slope and value of current throughthe MTJ 22 is adjusted (shown in graph of FIG. 1) by the circuit 300.The resistors 304 and 306 have varying levels of current flowing throughthem based on temperature in that the current through one of themincreases as a result of rising temperature while the current throughthe other decreases. A combination of these resistors provides adesirable slope for the curves 14 and 16 of graph 10. Current throughthe transistors of circuit 342 is proportional to the current throughthe resistor 326. The transistors of circuit 342 each function as acurrent mirror. The transistors of circuit 340, A-D, function asswitches resulting from the coupling of the transistors' respectivegates to substantially ground or Vdd (‘on’ and ‘off’).

Analogous to the embodiment of FIG. 2, the circuit 302 determines theslope of the curves shown in FIG. 1 with the goal of the circuit 300being to mimic the curve 12. Assuming the resistor 304 to be of type Aand resistor 306 to be of type B, the slope, i.e. ‘n’, is determined bythe following:

Rn=R _(A) R _(B)/(R _(A) +R _(B))  Eq. (2)

where ‘R_(A)’ is the resistance of resistor 304 and ‘R_(B)’ is theresistance of resistor 306. ‘Rn’ represents the voltage across MTJ22/the current flowing through the MTJ 22. Thus, the resistances of theresistors 304 and 306 determine the slope of the curve current vs.temperature for the MTJ 22. The voltage across MTJ 22 accordinglyfollows the slope of current versus temperature. To move the curve upand down, as in the embodiment of FIG. 2, the circuits 340 and 342 areemployed and the discussion above relative to the circuits 40 and 42apply to these circuits as well. That is, the circuit 340 includes anumber of transistors that each acts as a switch to turn ‘on’ and ‘off’each of the transistors of circuit 342 thereby causing a selectablelevel of current to flow through the MTJ 22. The transistor 326 servesas a current mirror and mirrors the current through the circuit 342, asdoes the transistor 324 relative to the circuit 340. Accordingly, as thecircuit 20 of FIG. 2, circuit 300 of FIG. 3 adjusts the slope and thelevel of the curve representing current versus temperature of MTJ 22.

FIG. 4 shows a circuit 400 for adjusting the current flow through a MTJ,in accordance with yet another embodiment of the invention. The circuit400 is shown to include the resistor 402, the diode 404, theanalog-to-digital converter 406, the digital current controller 408 andthe digital-to-analog converter 410. The circuit 400 is shown coupled tothe MTJ 22.

The Vbe across the diode 412 drops as temperature increases. Thisvoltage is then converted from analog to digital form by the converter406 where temperature fluctuations are represented digitally to thecontroller 408. Temperature is actually represented by a voltage at thenode 412 where resistor 402 and diode 404 are coupled. This is becausevoltage across the diode is a function of temperature. The controller408 converts the temperature to current values required by the MTJ. Theconverter 410 then converts the digital representation of current toanalog and provides the same to the MTJ 22. Thus, current flowingthrough the MTJ 22 is controlled by the circuit 400 but what makes theembodiment of circuit 400 different than the embodiments of FIGS. 1 and2 is that it measures temperature and adjusts the current accordingly.Therefore, a need for adjusting the slope and the up and down of thecurves of FIG. 1 are unnecessary. Current through the MTJ 22 is thuscontrollable and changed according to the sensed temperature, asrepresented by the voltage at node 412.

In the embodiment of FIG. 4, temperature is monitored on the chip thathouses an array of MTJs and used to produce proper current digitally.Temperature monitor generates an analog signal, such as that at node412. The Vbe across the diode 404 changes with temperature and thereforefunctions as a temperature sensor. As temperature rises, the currentthrough the diode 404 increases. Based on the change in current-increasethrough the diode 404, the voltage at node 50 drops. This voltage isdigitized and used to adjust current through the MTJ 22.

While a number of embodiments for controlling current as a function offluctuations in temperature are disclosed herein, it is understood thatthese are merely exemplary embodiments and others are contemplated.

Although the invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall such alterations and modification as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A circuit comprising: an MTJ formed on a chip; adiode circuit located on said chip and operable to measure a temperatureof said chip; and a device coupled to said diode circuit and operable togenerate a write current for programming said MTJ according to saidtemperature.
 2. The circuit of claim 1, wherein said device is a currentsource controlled by said diode circuit.
 3. The circuit of claim 2,wherein said current source is operable to determine a slope defined bya change in said write current through said MTJ versus a change in saidtemperature, said current source further operable to use said slope toadjust said write current.
 4. The circuit of claim 2, wherein saidcurrent source is operable to measure a change in a level of said writecurrent and to adjust said write current accordingly.
 5. The circuit ofclaim 1, wherein said device is a current generator.
 6. A circuitcomprising: an MTJ formed on a chip; a temperature-sensitive resistorlocated on said chip and operable to measure a temperature of said chip;and a device coupled to said temperature-sensitive resistor and operableto generate a write current for programming said MTJ according to saidtemperature.
 7. The circuit of claim 6, wherein said device is a currentsource.
 8. The circuit of claim 7, wherein said current source isoperable to determine a slope defined by a change in said write currentthrough said MTJ versus a change in said temperature, said currentsource further operable to use said slope to adjust said write current.9. The circuit of claim 7, wherein said current source is operable tomeasure a change in a level of said write current and to adjust saidwrite current accordingly.
 10. The circuit of claim 6, wherein saiddevice is a current generator.
 11. A circuit comprising: an MTJ formedon a chip; a temperature sensor located on said chip and operable tomeasure a temperature of said chip; a device coupled to said temperaturesensor and operable to generate a write current for programming said MTJaccording to said temperature; and an analog-to-digital converter,wherein said device is coupled to said MTJ.
 12. The circuit of claim 11,wherein said device is a current source controlled by said temperaturesensor.
 13. The circuit of claim 12, wherein said current source isoperable to determine a slope defined by a change in said write currentthrough said MTJ versus a change in said temperature, said currentsource further operable to use said slope to adjust said write current.14. The circuit of claim 12, wherein said current source is operable tomeasure a change in a level of said write current and to adjust saidwrite current accordingly.
 15. The circuit of claim 11, wherein saiddevice is a current generator.